1. Field of Invention
This invention relates to pickup coils used for generating current density images of the spatial distribution of two and three dimensional currents from the magnetic field produced by the currents. More specifically, the invention is directed to such coils having the spacing between multiple turns adjusted to provide a source-image transfer function with specific characteristics which improve the quality and spatial resolution of the image produced. The invention has particular application to the pickup coils used in the flux transformers of superconducting quantum interference devices (SQUIDs).
2. Background Information
It is known that an image of a two-dimensional current distribution can be reconstructed from measurements of its magnetic field using a magnetometer. This imaging technique can be useful in many applications such as noninvasive study of biological currents or nondestructive testing of electronic micro-circuits. There are several factors that determine the quality of the image: the magnitude and spatial extent of the current distribution, the noise of the magnetometer, the distance of the magnetometer to the current distribution, and the size of the magnetometer pickup coil. This patent is directed to the last of these factors, coil size, and presents a new optimized magnetometer coil design.
Traditionally magnetometer pickup coils are made of N turns of wire, each with the same radius, a. A number of different criteria have been used to optimize magnetometer pickup coil design. One of the most common is to determine the coil radius and number of turns that maximize the magnetometer signal-to-noise ratio for detecting an electric dipole in a conducting medium or a magnetic dipole in a non-magnetic medium. While this approach is useful, it does not take into account the spatial resolution achieved when imaging an extended source, an aspect of magnetometry that is growing in importance as investigators try to map localized current distributions with high spatial resolution.
A technique known as apodization is utilized in the field of optics to improve the spatial resolution of telescopes. A telescopic image of a distant object does not appear as a point but as a diffraction pattern, with the spatial extent of the pattern depending on the aperture of the telescope. A circular aperture will produce a diffraction pattern consisting of concentric rings. This ring system, which reduces the quality of the optical image, can be removed by apodization. The aperture function can be modified by placing a tinted glass over the aperture of the telescope with the opaqueness of the glass increasing from the center of the aperture to the edge. This modification eliminates the abrupt change in the aperture function at the edge of the aperture. The resulting diffraction pattern (which is just the square of the Fourier transform of the aperture function) is changed, and by a judicious choice of the aperture function, the ring system from the diffraction pattern can be greatly reduced or eliminated, thereby increasing the spatial resolution of the telescopic image.
A form of apodization has been applied to coils used in magnetic resonance imaging to make the coils easier to wind by reducing oscillations in the current density required to produce a magnetic field with a uniform gradient in a given space.
These coils are used for generating the magnetic field for the magnetic resonance imaging process and not to generate an image. Such coils are not at all suitable for generating current density images of two or three dimensional currents since they are specifically designed to provide a uniform magnetic field.
There is a need therefore for a magnetometer pickup coil for generating improved current density images of the spatial distribution of two and three dimensional currents from the magnetic fields produced by the currents.
More particularly, there is a need for such a magnetometer pickup coil with improved resolution.
There is also a need for such a magnetometer pickup coil with an improved signal-to-noise ratio.
There is a related need for providing such a pickup coil in which the size of the coil can be reduced while the number turns can be increased without a large increase in coil inductance.
There is in addition a particular need for such a low inductance coil for use in coupling to a thin film SQUID.